Arterial thrombosis is a major cause of illness and death. At the site of the vessel disease, blood platelets aggregate and form a major component of these thrombi. The platelets on the periphery of the thrombus may break away and return to the circulation, and this reversible activation reaction could result in significant membrane changes. The aim of this proposal is to examine four specific membrane changes which may result from platelet activation: actin polymerization causing decreased membrane deformability, fragmentation of this less deformable membrane to cause the loss of membrane microparticles, surface binding of endogenous secreted proteins, and changes of membrane-bound immunoglobulin. Our experiments will measure these changes (a) following activation by thrombin and (b) during the spontaneous clotting of whole blood, and we will correlate these in vitro results with studies of the platelets and plasma from patients with thrombotic disease. The polymerization of platelet actin will be measured by dexoyribonuclease inhibition. Microparticles of the platelet membrane will be quantified in plasma by a radioimmunoassay using a monoclonal antibody to an intrinsic membrane glycoprotein. The binding sites of endogenous secreted proteins on the membrane surface and their role in platelet adhesion will be defined. The effect of activation on the platelet IgG concentration and its subcellular distribution will be studied, specifically investigating the possibility that sialic acid loss during platelet activation may lead to IgG binding, with asialoglycoprotein Ib being a "senescent antigen" for naturally occurring autologous antibodies. Multiple reversible platelet contact interactions may result in an increase of polymerized actin, a progressive loss of membrane material, and the accumulation of immunoglobulin. This could occur in the normal circulation as a function of endothelial support causing the structural and functional changes of senescent platelets. These cycles of reversible interactions could be accelerated in patients with thrombotic disease, and measurement of these membrane changes of circulating platelets should allow the recognition of patients who have an increased risk for thrombosis.